1. Field of the Invention
The present invention relates to a circuit for, in a digital demodulator operable to perform demodulation of symbol values from a received signal, regenerating a symbol clock which defines a timing of performing a determination on the symbol values, and a receive using the same.
2. Description of the Related Art
In a configuration designed to transmit a clock from a base station, such as conventional mobile communication using a base station, synchronization can be obtained by receiving the clock from the base station. However, in a wireless unit where a call is made only when necessary, such as a 400 MHz band digital wireless unit, in order to demodulate symbol values after receiving a modulated signal by means of FSK (Frequency Shift Keying) or the like, it is necessary to firstly regenerate the aforementioned symbol clock. Heretofore, common symbol-clock regeneration has been performed by allowing a signal obtained by squaring a demodulated signal to pass through a band-pass filter having a center frequency at a clock frequency. Then, in order to correct a phase difference caused by the band-pass filter, there has been employed a method of performing the correction, for example, by detecting the phase difference based on a zero cross point where a positive/negative sign of a clock component is changed. In 2-level modulated waves by means of 2-level FSK or the like, a demodulated signal is formed in a shape close to a sinusoidal wave, so that it becomes possible for the above conventional technique to extract a clock component by squaring a demodulated signal, as described above. However, in multi-level modulated waves by means of 4-level FSK or the like, a demodulated signal has a plurality of amplitudes, so that a clock component cannot be extracted even if the square calculation is performed. Therefore, as a conventional technique for solving such a problem, JP 10-4436A is cited, for example.
In JP20-4426A, in cases where there is a transition between sample values having different absolute values, as in the above 4-level FSK, a curve representing the transition deviates from a timing of a clock having a proper zero-crossing timing. Thus, a clock regeneration circuit is shown which is operable, after detecting multi-level modulated waves, to perform oversampling at a clock faster than the symbol clock, and make a judgment on delay or advance of a regenerated clock, depending on whether a cross timing signal output at a timing when a data sequence obtained by the sampling crosses a threshold value is in a first half of a symbol interval or in a last half of the symbol interval.
Meanwhile, it is known that, if the Nyquist second criterion is not satisfied, a deviation occurs in a timing when an inter-state transition curve crosses a given voltage. When communication is performed in a narrow band, a situation arises where the Nyquist second criterion is not satisfied, so that, in the above conventional technique, a deviation occurs in a cross point with the threshold value, in each symbol. Consequently, it cannot be determined whether a phase of the regenerated clock is delayed or advanced, to cause difficulty in stable clock regeneration. Particularly, if the cross timing signal is output at a timing during state transition of the clock, a 180° delay and a 180° advance will not be able to be distinguished from each other. Moreover, as the number of threshold values is increased, a calculation amount of a timing calculation is increased.